1. Field of the Invention
This invention relates to a pipetting and diluting apparatus and, more particularly to apparatus for dosing of small quantities of liquids. The apparatus is particularly adapted for use with interchangeable pump modules and permits direct digital volume setting in milliliter or microliter units.
2. Description of the Prior Art
A large number of analyses conducted in laboratory practice of various fields of medicine, biology, chemistry and industry, frequently require the necessity that liquid samples with different viscosities have to be pipetted quantitatively in the microliter or milliliter range and diluted with reagents. For this kind of sample handling liquid delivery equipments having a set of two pumps are usually used in connection with values embodied into the pump modules and provided with an inlet and outlet port. Under suitable valve positions both, the sample and reagent, not necessarily of the same volume, will be sucked in simultaneously and discharged sequentially. A high accuracy is required related to volume setting and metering of very small quantities of a liquid.
Interchangeable piston displacement pump modules with different piston diameter are generally applied in order to cover a more extended volume range whereby a constant piston path is always correlated to the different total volumes of the pump cylinders. Furthermore, it is known to prior art that each of the pistons may be driven by means of a stepper motor under insertion of suitable transmission elements. Thus the total piston path within the pump cylinder will functionally correspond to a definite number of steps generated by the stepper motor in conjunction with the stepper driver.
A dosing apparatus is known and described in German Pat. No. 23 29 136 utilizing an interchangeable piston pump unit with direct volume control in fixed incremental steps. Push buttons activating mechanical stops provide means of volume control. Moreover each of the plugged-in piston pump units is provided with volume reading means as a digital label of which the numbers are positioned to the corresponding buttons. In such a way it is possible to set directly different liquid volumes, which are indicated on the appropriate pump unit. Equipments of this kind having direct volume setting are disadvantageous since each pump module has to be provided with a digital label which is specifically coordinated to their total volume and permits only in conjunction with the corresponding buttons a rough incremental graduation of setting related to the different volumes.
Furthermore a "Digital Diluter Dispenser" manufactured by the company Hamilton and described in a publication of the company Micromesure AG, Bonaduz, Switzerland, contains two interchangeable pump modules and can be applied as a diluter or as a pipetter by the use of interchangeable valve blocks. The piston of the Hamilton gastight syringes is driven by a stepper motor under insertion of a gear belt transmission and a gear rack.
Volume control is carried out as a percentage of total volume of the syringe on a two-digit switch which is a distinct disadvantage since it is a source of errors which cannot be excluded. Furthermore liquid quantities which correspond to the 100% capacity of the pump module can neither be taken in nor be dispensed.
In connection with a large number of analyses performed in a laboratory, microliter or milliliter quantities of liquid samples with different viscosity have frequently to be dosed. In order to guarantee a small dead volume in the lines of tubing and the valve blocks of the pipetter/diluter devices, the inside diameter of tubes and bores of valve bodies is kept as small as possible.
Since sample throughput is of high concern in the lab, the time of a complete cycle for aqueous samples should not exceed 5 seconds. Therefore a very high linear speed of the liquids becomes effective in the lines of tubing during the intake and discharge stroke and can reach values up to several centimeters per second. The known device of Hamilton using a stepper motor drive has the further disadvantage of a fixed slope of the start and stop characteristic of the piston motion controlled according to an established exponential function which is independent of the total volume of the pump module and of the preselected piston speed. In association therewith, a cavitation of liquids to be dosed can occur due to a temporary negative pressure within the pump body during the intake stroke and simultaneously an extension of the tubes during the discharge stroke.
Both (double-pump) systems mentioned before are not able to pipette small quantities of sample liquids quantitatively without adding diluents or reagents through the sample pump. A last drop remains on the tip of the pipet tubing due to adhesion and surface tension.